A normal ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane (eardrum) 102, which moves the ossicles of the middle ear 103 (malleus, incus, and stapes) that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow organ wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ear's ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, various types of hearing prostheses have been developed. For example, when a hearing impairment is related to the operation of the middle ear 103, a conventional hearing aid or a middle ear implant (MEI) device may be used to provide acoustic-mechanical vibration to the auditory system.
FIG. 1 also shows some components in a typical MEI arrangement where an external audio processor 111 processes ambient sounds to produce an implant communications signal that is transmitted through the skin by external transmitter 107 to an implanted receiver 108. Receiver 108 includes a receiver coil that transcutaneously receives the implant communications signal which is then demodulated into transducer stimulation signals which are sent over leads 109 through a surgically created channel in the temporal bone to a floating mass transducer (FMT) 110 secured to the incus bone in the middle ear 103. The transducer stimulation signals cause drive coils within the FMT 110 to generate varying magnetic fields which in turn vibrate a magnetic mass suspended within the FMT 110. The vibration of the inertial mass of the magnet within the FMT 110 creates vibration of the housing of the FMT 110 relative to the magnet. This vibration of the FMT 110 is coupled to the incus in the middle ear 103 and then to the cochlea 104 and is perceived by the user as sound. See U.S. Pat. No. 6,190,305, which is incorporated herein by reference.
FIG. 2A shows an FMT 110 ideally implanted so that its end drive surface 203 generates a mechanical stimulation signal that optimally drives the round window membrane 202 to vibrate the fluid within the scala tympani 201. In some patients, it may be difficult or impossible to implant the FMT 110 as shown in FIG. 2A with its cylindrical axis perpendicular to the round window membrane 202. When the patient's anatomy does not allow placement of the FMT 110 perpendicular to the round window membrane 202, FIG. 2B shows an FMT 110 that drives the round window membrane 202 via a vibroplasty coupling cap 204 within a round drive surface that drives the round window membrane 202 at an angle to the longitudinal axis of the FMT 110.
But implanting the FMT 110 with such a coupling cap 204 can be difficult. Because the FMT 110 and the coupling cap 204 are so small, they can be hard to handle and manipulate with standard non-custom surgical tools. It can be difficult for the surgeon to correctly place the coupling cap 204 coaxially onto the FMT 110 at a right angle. In other cases, the coupling cap 204 is initially placed correctly onto the FMT 110, but then gets pushed askew or off during the surgery (the holding force of the clamping fingers on the coupling cap 204 is limited). Or the surgeon may accidentally damage the FMT 110 and/or the coupling cap 204. These problems can arise due to various factors, for example, stress or poor lighting, and they can become quite time consuming to correct.
In addition, the arrangement as shown in FIG. 2 where the coupling cap 204 has clamping fingers that fit over the FMT 110 increases the outside diameter of the resulting arrangement which undesirably increases the volume needed for implantation.
Nor is it the case that a conventional adhesive material can simply be added to increase the bonding force between the FMT 110 and the coupling cap 204. There is no way to apply the same amount of adhesive material in the same repeatable way to bond the FMT 110 and the coupling cap 204 so as to provide a consistent and predictable effect in the operating performance of the implanted FMT 110. The adhesive material also has elastic properties that generated undesired linear and non-linear damping in the mechanical stimulation signal that is coupled between the FMT 110 and the coupling cap 204. In addition, evaporating the dilutant component of the adhesive material during surgery takes significant time and requires complicated sterile-safe procedures and other procedures that ensure that the toxic dilutant safely and completely evaporates. The only acceptable adhesive for use in an implantation application is fibrin glue, which is only intended for use with tissue, not mechanical components such as an FMT 110. A further challenge is to achieve an acceptably precise alignment of the coupling cap 204 with the FMT 110.